The reactivity of gas-phase ground-state calcium atoms with O-2 is reported from 296 to 623 K, Calcium atoms were produced by the photodissociation of Ca(FOD)(2) [where FOD represents the 6,6,7.7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate ion] and detected by laser-induced fluorescence. Calcium recombines with O-2 to form CaO2 in a reaction that exhibits a positive temperature dependence and is significantly in the falloff region even at pressures as low as 10 Torr. Ab initio quantum calculations are used to show that the reaction most likely proceeds on a tripler surface, where the Ca atom inserts into the O-O bond to form a triplet dioxide (B-3(2)) With a very low frequency bending mode. The surprisingly large rate coefficient at elevated temperatures is explained by the correspondingly high density of rovibrational states, and the positive temperature dependence by a small barrier in the entrance channel at the crossing point between the covalent and ionic diabats. RRKM theory, fitted to the experimental data, predicts the following expression for the rate coefficient from 140 to 1000 K and 10(-5) to 10(3) Torr: log(k(rec.0)/cm(6) molecule(-1) s(-1)) = -57.20 + 19.70 log T - 3.410(log T)(2), k(rec,infinity) = 1.36 x 10(-10) exp(-1020/T) cm(3) molecule(-1) s(-1), and F-c = 0.67.